Thin hollow backlights with beneficial design characteristics

ABSTRACT

An edge-lit backlight comprises a front and back reflector forming a hollow light recycling cavity having a cavity depth H and an output region of area Aout, and one or more light sources disposed proximate a periphery of the backlight to emit light into the light recycling cavity. The light sources have an average plan view source separation of SEP collectively having an active emitting area Aemit, wherein a first parameter equals Aemit/Aout and a second parameter equals SEP/H. The first parameter is in a range from 0.0001 to 0.1, and by the second parameter is in a range from 3 to 10. The front reflector has a hemispherical reflectivity for unpolarized visible light of R f   hemi , and the back reflector has a hemispherical reflectivity for unpolarized visible light of R b   hemi , and R f   hemi *R b   hemi  is at least 0.70.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing under 35 U.S.C. §371 ofPCT/US2008/064096, filed on May 19, 2008, which claims priority toProvisional Application No. 60/939,084, filed on May 20, 2007, thedisclosure of which is incorporated by reference in its/their entiretyherein.

RELATED APPLICATIONS

The following co-owned and copending PCT Patent Applications areincorporated herein by reference: PCT Patent Publication No.WO2008/144656; PCT Patent Publication No. WO2008/144644; PCT PatentPublication No. WO2008/147753; and PCT Patent Publication No.WO2008/144650.

FIELD

The present disclosure relates to extended area light sources suitablefor illuminating a display or other graphic from behind, commonlyreferred to as backlights. The disclosure is particularly suited, butnot necessarily limited, to backlights that emit visible light ofsubstantially only one polarization state.

BACKGROUND

Historically, simple backlight devices included only three maincomponents: light sources or lamps, a back reflector, and a frontdiffuser. Such systems are still in use for general purpose advertisingsigns and for indoor lighting applications.

Over recent years, refinements have been made to this basic backlightdesign by adding other components to increase brightness or reduce powerconsumption, increase uniformity, and/or reduce thickness. Therefinements have been fueled by demands in the high-growth consumerelectronics industry for products that incorporate liquid crystaldisplays (LCDs), such as computer monitors, television monitors, mobilephones, digital cameras, pocket-sized MP3 music players, personaldigital assistants (PDAs), and other hand-held devices. Some of theserefinements, such as the use of solid light guides to allow the designof very thin backlights, and the use of light management films such aslinear prismatic films and reflective polarizing films to increaseon-axis brightness, are mentioned herein in connection with furtherbackground information on LCD devices.

Although some of the above-listed products can use ordinary ambientlight to view the display, most include a backlight to make the displayvisible. In the case of LCD devices, this is because an LCD panel is notself-illuminating, and thus is usually viewed using an illuminationassembly or backlight. The backlight is situated on the opposite side ofthe LCD panel from the viewer, such that light generated by thebacklight passes through the LCD to reach the viewer. The backlightincorporates one or more light sources, such as cold cathode fluorescentlamps (CCFLs) or light emitting diodes (LEDs), and distributes lightfrom the sources over an output area that matches the viewable area ofthe LCD panel. Light emitted by the backlight desirably has sufficientbrightness and sufficient spatial uniformity over the output area of thebacklight to provide the user with a satisfactory viewing experience ofthe image produced by the LCD panel.

LCD panels, because of their method of operation, utilize only onepolarization state of light, and hence for LCD applications it isimportant to know the backlight's brightness and uniformity for light ofthe correct or useable polarization state, rather than simply thebrightness and uniformity of light that may be unpolarized. In thatregard, with all other factors being equal, a backlight that emits lightpredominantly or exclusively in the useable polarization state is moreefficient in an LCD application than a backlight that emits unpolarizedlight. Nevertheless, backlights that emit light that is not exclusivelyin the useable polarization state, even to the extent of emittingrandomly polarized light, are still fully useable in LCD applications,since the non-useable polarization state can be easily eliminated by anabsorbing polarizer provided at the back of the LCD panel.

LCD devices generally fall within one of three categories, andbacklights are used in two of these categories. In a first category,known as “transmission-type,” the LCD panel can be viewed only with theaid of an illuminated backlight. That is, the LCD panel is configured tobe viewed only “in transmission,” with light from the backlight beingtransmitted through the LCD on its way to the viewer. In a secondcategory, known as “reflective-type,” the backlight is eliminated andreplaced with a reflective material, and the LCD panel is configured tobe viewed only with light sources situated on the viewer-side of theLCD. Light from an external source (e.g., ambient room light) passesfrom the front to the back of the LCD panel, reflects off of thereflective material, and passes again through the LCD on its way to theviewer. In a third category, known as “transflective-type,” both abacklight and a partially reflective material are placed behind the LCDpanel, which is configured to be viewed either in transmission if thebacklight is turned on, or in reflection if the backlight is turned offand sufficient ambient light is present.

Backlights described in the detailed description below can generally beused both in transmission-type LCD displays and in transflective-typeLCD displays.

Besides the three categories of LCD displays discussed above, backlightscan also fall into one of two categories depending on where the internallight sources are positioned relative to the output area of thebacklight, where the backlight “output area” corresponds to the viewablearea or region of the display device. The “output area” of a backlightis sometimes referred to herein as an “output region” or “outputsurface” to distinguish between the region or surface itself and thearea (the numerical quantity having units of square meters, squaremillimeters, square inches, or the like) of that region or surface.

In “edge-lit” backlights, one or more light sources are disposed—from aplan-view perspective—along an outer border or periphery of thebacklight construction, generally outside the area or zone correspondingto the output area. Often, the light source(s) are shielded from view bya frame or bezel that borders the output area of the backlight. Thelight source(s) typically emit light into a component referred to as a“light guide,” particularly in cases where a very thin profile backlightis desired, as in laptop computer displays. The light guide is a clear,solid, and relatively thin plate whose length and width dimensions areon the order of the backlight output area. The light guide uses totalinternal reflection (TIR) to transport or guide light from theedge-mounted lamps across the entire length or width of the light guideto the opposite edge of the backlight, and a non-uniform pattern oflocalized extraction structures is provided on a surface of the lightguide to redirect some of this guided light out of the light guidetoward the output area of the backlight. (Other methods of gradualextraction include using a tapered solid guide, where the sloping topsurface causes a gradual extraction of light as the TIR angle is, onaverage, now reached by greater numbers of light rays as the lightpropagates away from the light source.) Such backlights typically alsoinclude light management films, such as a reflective material disposedbehind or below the light guide, and a reflective polarizing film andprismatic Brightness Enhancement Films (BEF) film(s) disposed in frontof or above the light guide, to increase on-axis brightness.

In the view of Applicants, drawbacks or limitations of existing edge-litbacklights include the following: the relatively large mass or weightassociated with the light guide, particularly for larger backlightsizes; the need to use components that are non-interchangeable from onebacklight to another, since light guides must be injection molded orotherwise fabricated for a specific backlight size and for a specificsource configuration; the need to use components that requiresubstantial spatial non-uniformities from one position in the backlightto another, as with existing extraction structure patterns; and, asbacklight sizes increase, increased difficulty in providing adequateillumination due to limited space or “real estate” along the edge of thedisplay, since the ratio of the circumference to the area of a rectangledecreases linearly (1/L) with the characteristic in-plane dimension L(e.g., length, or width, or diagonal measure of the output region of thebacklight, for a given aspect ratio rectangle).

In “direct-lit” backlights, one or more light sources are disposed—froma plan-view perspective—substantially within the area or zonecorresponding to the output area, normally in a regular array or patternwithin the zone. Alternatively, one can say that the light source(s) ina direct-lit backlight are disposed directly behind the output area ofthe backlight. Because the light sources are potentially directlyviewable through the output area, a strongly diffusing plate istypically mounted above the light sources to spread light over theoutput area to veil the light sources from direct view. Again, lightmanagement films, such as a reflective polarizer film, and prismatic BEFfilm(s), can also be placed atop the diffuser plate for improved on-axisbrightness and efficiency. Large area LCD applications tend to usedirect-lit backlights because they are not constrained by the 1/Llimitation of edge-lit backlights and because of the weight associatedwith solid light guides.

In the view of Applicants, drawbacks or limitations of existingdirect-lit backlights include the following: inefficiencies associatedwith the strongly diffusing plate; in the case of LED sources, the needfor large numbers of such sources for adequate uniformity andbrightness, with associated high component cost and heat generation; andlimitations on achievable thinness of the backlight beyond which lightsources produce non-uniform and undesirable “punchthrough,” where abright spot appears in the output area above each source.

In some cases, a direct-lit backlight may also include one or some lightsources at the periphery of the backlight, or an edge-lit backlight mayinclude one or some light sources directly behind the output area. Insuch cases, the backlight is considered “direct-lit” if most of thelight originates from directly behind the output area of the backlight,and “edge-lit” if most of the light originates from the periphery of theoutput area of the backlight.

BRIEF SUMMARY

The present application discloses, inter alia, edge-lit backlights thatinclude a front and back reflector that form a hollow light recyclingcavity. The recycling cavity has an output area Aout and acharacteristic cavity depth H between the cavity output area and thecavity back surface. One or more light sources are disposed proximate aperiphery of the backlight to emit light into the light recyclingcavity. These light sources can be described by their geometricarrangement relative to one another, including the distances betweenthem, and the way they may be aggregated. For example, planar arrays oflight sources can have an average plan view source aggregation of “SEP,”and collectively the light sources have an active emitting area Aemit.The backlights are characterized by a first parameter being in a rangefrom 0.0001 to 0.1 and a second parameter being in a range from 3 to 50,where the first parameter equals Aemit/Aout, and the second parameterequals SEP/H. The light sources may be arranged predominantly at aperiphery of the output area to provide an edge-lit backlight, orarranged predominantly within the space of the output area to provide adirect-lit backlight. Backlights within the recited first and secondparameter ranges can have any suitable physical size, large or small.For example, such a backlight may be on the order of an inch in lateraldimension (e.g., diagonal measure of a rectangular output area), and insuch case may be one of many partitioned zones in a larger zonedbacklight.

The application also discloses edge-lit backlights having a front andback reflector that form a hollow recycling cavity, and that can berelatively large regardless of their first and second parameter values.The front reflector, which is partially transmissive, provides an outputarea of the backlight that may be generally rectangular in shape. Adiagonal measure of the rectangular shape can be any suitable value. Insome embodiments, the diagonal can be at least 12 inches (300 mm). Thehollow cavity can advantageously reduce the mass of the backlightrelative to an edge-lit backlight that uses a solid light guide.

The application also discloses backlights in which light is distributedso effectively and efficiently in transverse or lateral directions thatthe backlights are highly resistant to the visible effects of sourcefailure and/or source-to-source color variability. The brightnessuniformity over the output area of such a backlight is only modestlydiminished when individual light sources within the backlight degrade,fail, or are turned off. For example, backlights are disclosed in whicha number N of light sources emit light into a recycling cavity formedbetween a front and back reflector, with some of the emitted lightpassing through the front reflector to form the output area of thebacklight. The number N can be at least 8, and the N light sourcesinclude a subset of M light sources that are adjacent to each other,where M is at least 10% of N, or is at least 2, or both. The backlightmaintains adequate brightness uniformity over the output area both whenall N light sources are energized and when all of the M light sourcesare selectively turned off. Because of the excellent lateral ortransverse light distribution (“light mixing”) in the recycling cavity,backlights such as this are also typically less sensitive to problemsassociated with color variability among LED sources that are allnominally the same color. This color variability typically requires thatLEDs be binned.

In many cases, it is desirable to provide very high recycling cavities,where the front reflector has a hemispherical reflectivity for visibleunpolarized light of R^(f) _(hemi), the back reflector has ahemispherical reflectivity for visible unpolarized light of R^(b)_(hemi), and the product R^(f) _(hemi)*R^(b) _(hemi) is at least 0.70.For example, if the back reflector has an R^(b) _(hemi) of 98%, then thefront reflector has an R^(f) _(hemi) of at least 71.4%. If the frontreflector is optionally fabricated to reflect and transmit differentpolarization states differently, it may then have a hemisphericalreflectivity for visible light of a first polarization state of 98%, anda hemispherical reflectivity for visible light of a second polarizationstate (e.g., the useable polarization state) orthogonal to the firstpolarization state of 78%. In such a case, the second or useablepolarization state is predominantly reflected by the front reflectoreven though it is preferentially transmitted in comparison to the firstpolarization state.

It is also often desirable to ensure that the amount of lighttransmitted through the front reflector is substantially greater thanthe amount of light transmitted or otherwise lost (e.g., by absorption)by the back reflector. Thus, for example, the ratio of (1−R^(f)_(hemi))/(1−R^(b) _(hemi)) is at least 10.

Besides the front and back reflectors, highly reflective and low lossside reflectors are preferably provided to yield a substantially closedor sealed reflecting cavity, and losses associated with the lightsources are kept to minimal levels by, for example, maintaining a verysmall ratio of collective source area to backlight output area. In someinstances, highly reflective and low loss side reflectors can aid in thelateral and transverse transport and mixing of light in a high recyclingcavity.

These and other aspects of the present application will be apparent fromthe detailed description below. In no event, however, should the abovesummaries be construed as limitations on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the specification reference is made to the appended drawings,where like reference numerals designate like elements, and wherein:

FIG. 1 is a schematic side view of a generalized recycling backlight orsimilar extended area source;

FIGS. 2 a-c are top plan views of different light source arrangementswithin a recycling cavity;

FIG. 3 is a graph of a backlight parameter design space defined by twodimensionless parameters, Parameter A and Parameter B;

FIG. 4 is a graph of backlight parameter design space with pointsplotted corresponding to a variety of commercially available LCD displaydevices;

FIG. 5 is a perspective view of a surface, showing different planes ofincidence and different polarization states;

FIG. 6 is a front view of a backlight output area;

FIG. 7 is a schematic cross-section of a display system that includes abacklight in combination with an LCD panel;

FIG. 8 is a plan view of an LED cluster arrangement;

FIG. 9 is a graph showing average luminance for different reflectors anddifferent “effective reflectivities” of the back reflector; and

FIGS. 10 and 11 are graphs that plot the various backlight examples inthe Parameter A/Parameter B design space, with FIG. 10 plotting edge-litbacklights and FIG. 11 plotting direct-lit backlights.

DETAILED DESCRIPTION

It would be beneficial for next generation backlights to combine some orall of the following characteristics while providing a brightness andspatial uniformity that is adequate for the intended application: thinprofile; design simplicity, such as a minimal number of opticalcomponents and a minimal number of sources, and convenient sourcelayout; low weight; no use of or need for film components havingsubstantial spatial non-uniformities from one position in the backlightto another (e.g., no significant gradation); compatibility with LEDsources, as well as other small area, high brightness sources such assolid state laser sources; insensitivity to problems associated withcolor variability among LED sources that are all nominally the samecolor; to the extent possible, insensitivity to the burnout or otherfailure of a subset of LED sources; and the elimination or reduction ofat least some of the limitations and drawbacks mentioned in theBackground section above.

Whether these characteristics can be successfully incorporated into abacklight depends in part on the type of light source used forilluminating the backlight. CCFLs, for example, provide white lightemission over their long narrow active emissive areas, and thoseemissive areas can also operate to scatter some light impinging on theCCFL, such as would occur in a recycling cavity. The typical emissionfrom a CCFL, however, has an angular distribution that is substantiallyLambertian, which may be inefficient or otherwise undesirable in a givenbacklight design. Also, the emissive surface of a CCFL, althoughsomewhat diffusely reflective, also typically has an absorptive lossthat Applicants have found to be significant if a highly recyclingcavity is desired.

An LED die emits light in a near-Lambertian manner, but because of itsmuch smaller size relative to CCFLs, the LED light distribution can bereadily modified, e.g., with an integral encapsulant lens or reflectoror extractor to make the resulting packaged LED a forward-emitter, aside-emitter, or other non-Lambertian profile. Such non-Lambertianprofiles can provide important advantages for the disclosed backlights.However, the smaller size and higher intensity of LED sources relativeto CCFLs can also make it more difficult to produce a spatially uniformbacklight output area using LEDs. This is particularly true in caseswhere individual colored LEDs, such as arrangements of red/green/blue(RGB) LEDs, are used to produce white light, since failure to provideadequate lateral transport or mixing of such light can easily result inundesirable colored bands or areas. White light emitting LEDs, in whicha phosphor is excited by a blue or UV-emitting LED die to produceintense white light from a small area or volume on the order of an LEDdie, can be used to reduce such color non-uniformity, but white LEDscurrently are unable to provide LCD color gamuts as wide as thoseachievable with individual colored LED arrangements, and thus may not bedesirable for all end-use applications.

Applicants have discovered combinations of backlight design featuresthat are compatible with LED source illumination, and that can producebacklight designs that outperform backlights found in state-of-the-artcommercially available LCD devices in at least some respects. Thesebacklight design features include some or all of the following:

-   -   a recycling optical cavity in which a large proportion of the        light undergoes multiple reflections between substantially        coextensive front and back reflectors before emerging from the        front reflector, which is partially transmissive and partially        reflective;    -   overall losses for light propagating in the recycling cavity are        kept extraordinarily low, for example, both by providing a        substantially enclosed cavity of low absorptive loss, including        low loss front and back reflectors as well as side reflectors,        and by keeping losses associated with the light sources very        low, for example, by ensuring the cumulative emitting area of        all the light sources is a small fraction of the backlight        output area;    -   a recycling optical cavity that is hollow, i.e., the lateral        transport of light within the cavity occurs predominantly in        air, vacuum, or the like rather than in an optically dense        medium such as acrylic or glass;    -   in the case of a backlight designed to emit only light in a        particular (useable) polarization state, the front reflector has        a high enough reflectivity for such useable light to support        lateral transport or spreading, and for light ray angle        randomization to achieve acceptable spatial uniformity of the        backlight output, but a high enough transmission into the        appropriate application-useable angles to ensure application        brightness of the backlight is acceptable;    -   the recycling optical cavity contains a component or components        that provide the cavity with a balance of specular and diffuse        characteristics, the component having sufficient specularity to        support significant lateral light transport or mixing within the        cavity, but also having sufficient diffusivity to substantially        homogenize the angular distribution of steady state light within        the cavity, even when injecting light into the cavity only over        a narrow range of angles (and further, in the case of a        backlight designed to emit only light in a particular (useable)        polarization state, recycling within the cavity preferably        includes a degree of randomization of reflected light        polarization relative to the incident light polarization state,        which allows a mechanism by which non-useable polarized light is        converted into useable polarized light);    -   the front reflector of the recycling cavity has a reflectivity        that generally increases with angle of incidence, and a        transmission that generally decreases with angle of incidence,        where the reflectivity and transmission are for unpolarized        visible light and for any plane of incidence, and/or for light        of a useable polarization state incident in a plane for which        oblique light of the useable polarization state is p-polarized        (and further, the front reflector has a high value of        hemispheric reflectivity while also having a sufficiently high        transmission of application-useable light);    -   light injection optics that partially collimate or confine light        initially injected into the recycling cavity to propagation        directions close to a transverse plane (the transverse plane        being parallel to the output area of the backlight), e.g., an        injection beam having a full angle-width (about the transverse        plane) at half maximum power (FWHM) in a range from 0 to 90        degrees, or 0 to 60 degrees, or 0 to 30 degrees. In some        instances it may be desirable for the maximum power of the        injection light to have a downward projection, below the        transverse plane, at an angle with the transverse plane of no        greater than 40 degrees, and in other instances, to have the        maximum power of the injected light to have an upwards        projection, above the transverse plane towards the front        reflector, at an angle with the transverse plane of no greater        than 40 degrees.

Backlights for LCD panels, in their simplest form, consist of lightgeneration surfaces such as the active emitting surfaces of LED dies orthe outer layers of phosphor in a CCFL bulb, and a geometric and opticalarrangement of distributing or spreading this light in such a way as toproduce an extended- or large-area illumination surface or region,referred to as the backlight output area, which is spatially uniform inits emitted brightness. Generally, this process of transforming veryhigh brightness local sources of light into a large-area uniform outputsurface results in a loss of light because of interactions with all ofthe backlight cavity surfaces, and interaction with the light-generationsurfaces. Other approaches such as using direct-lit source architectureswith specified LED lenses to level the incident first bounce flux on thefront reflector can result in efficient, uniform brightness through thebacklight output surface, but these approaches can be very sensitive tothe exactly geometrical configuration of all of the backlightcomponents. To a first approximation, any light that is not delivered bythis process through the output area or surface associated with a frontreflector—optionally into a desired application viewer-cone (if any),and with a particular (e.g., LCD-useable) polarization state (if any)—is“lost” light.

We propose that any backlight containing a recycling cavity can beuniquely characterized by two essential parameters. In this regard,reference is made to the generalized backlight 10 shown in FIG. 1, inwhich a front reflector 12 and a back reflector 14 form a recyclingcavity 16. The backlight 10 emits light over an extended output area orsurface 18, which in this case corresponds to an outer major surface ofthe front reflector 12. The front and back reflectors 12, 14 are shownplane and parallel to each other, and coextensive over a transversedimension 13, which dimension also corresponds to a transverse dimensionsuch as a length or width of the output area 18.

In other embodiments, the front and back reflectors 12, 14 can benon-parallel, e.g., as is further described in co-owned U.S. patentapplication Ser. No. 61/030,767 entitled BACKLIGHTS HAVING SELECTEDOUTPUT LIGHT FLUX DISTRIBUTION AND DISPLAY SYSTEMS USING SAME. Thisnon-parallel relationship can be provided using any suitable technique.For example, one or both of the top and bottom reflectors can be formedinto non-planar shapes, the front and back reflectors can be positionedsuch that they are non-parallel, one or more structures can bepositioned on one or both of the front and back reflectors, or anycombination of these techniques can be utilized to provide anon-parallel relationship.

The front reflector reflects a substantial amount of light incident uponit from within the cavity, as shown by an initial light beam 20 beingreflected into a relatively strong reflected beam 20 a and a relativelyweaker transmitted beam 20 b. Note that the arrows representing thevarious beams are schematic in nature, e.g., the illustrated propagationdirections and angular distributions of the different beams are notintended to be completely accurate. Returning to the figure, reflectedbeam 20 a is strongly reflected by back reflector 14 into a beam 20 c.Beam 20 c is partially transmitted by front reflector 12 to producetransmitted beam 20 d, and partially reflected to produce another beam(not shown). The multiple reflections between the front and backreflectors help to support transverse propagation of light within thecavity, indicated by arrow 22. The totality of all transmitted beams 20b, 20 d, and so on add together incoherently to provide the backlightoutput.

For illustrative purposes, small area light sources 24 a, 24 b, 24 c areshown in alternative positions in the figure, where source 24 a is shownin an edge-lit position and is provided with a reflective structure 26that can help to collimate (at least partially) light from the source 24a. Sources 24 b and 24 c are shown in direct-lit positions, and source24 c would generally be aligned with a hole or aperture (not shown)provided in the back reflector 14 to permit light injection into thecavity 16. Reflective side surfaces (not shown, other than reflectivestructure 26) would typically also be provided generally at theendpoints of dimension 13, preferably connecting the front and backreflectors 12, 14 in a sealed fashion for minimum losses.

Any suitable technique can be used to seal together the front and backreflectors. For example, in some embodiments, a reflective caulk can beused to seal the front and back reflectors 12, 14 together. Thereflective caulk can provide a reflective barrier to prevent lightleakage from the backlight cavity and provide a mechanical bond tosecure together the front and back reflectors 12, 14. Any suitablereflective caulk can be utilized. For example, the caulk can include aclear matrix that is loaded with reflective particles and/or may containregions of differing refractive index. Suitable particles include TiO₂or ribbons or flakes of reflective materials, such as metal chips,metallized polymer films, nacreous pigments, or multilayer opticalfilms. The clear matrix may be of any material suitable to encapsulateand deliver the reflective elements along with providing sufficient bondstrength to the applied surfaces. Regions of differing refractive indexmay be formed through an immiscible blend chemistry, loading of glass orpolymer beads, or the injection of air or other dissimilar materialsinto the matrix. The caulk may be dispensed through a nozzle to form abead or other shaped profile. A convex or concave cross-sectionalprofile may offer additional design options for managing the lightinside the backlight cavity. The caulk may be thermally or opticallycured. In some embodiments, the caulk may include an epoxy type systemwith suitable viscosity for dispensing and shape retention.

In some direct-lit embodiments, generally vertical reflective sidesurfaces may actually be thin partitions that separate the backlightfrom similar or identical neighboring backlights, where each suchbacklight is actually a portion of a larger zoned backlight. Lightsources in the individual sub-backlights can be turned on or off in anydesired combination to provide patterns of illuminated and darkenedzones for the larger backlight. Such zoned backlighting can be useddynamically to improve contrast and save energy in some LCDapplications. The reflective partitions between zones may not extendcompletely to the top reflector, but may be separated therefrom by a gapthat is sized to minimize the visibility of zone boundaries (from theperspective of a viewer) while also optimizing zone-to-zonebleedthrough.

Returning to our two-parameter discussion, the first parameter, referredto herein as Parameter A, relates the total emitting source area to thebacklight output area. Thus, Parameter A is the ratio of the total areaof all emitting light source surfaces (referred to herein as “Aemit”) tothe area of the output surface of the backlight (referred to herein as“Aout”). In the usual case of a rectangular-shaped output region, thearea Aout is simply the rectangle's length times its width. For a givenbacklight, the total area of the light source surfaces can be determinedby summing the active area of the light sources. For example, aLumileds™ LXHL-PM09 green LED, considered a “large die” LED, has a diesurface area (one large top surface and four smaller side surfaces) ofabout 1 mm². A Nichia Rigel NFSG036B green LED, considered a “small die”LED, has a die surface area of about 0.09 mm². A backlight having anarray that consists of 65 “large die” LED clusters—one red, one blue,and one green, whose outputs are balanced to produce white light whencombined—would have a total light source surface area ofAemit=65 clusters×3 dies/cluster×1 mm²/die=195 mm².

For a CCFL-based backlight, the total light-generation surface area ismerely the total surface area of the light-emitting phosphor layer perbulb, times the number of bulbs illuminating the cavity. For example, abacklight containing 16 CCFL bulbs, each 820 mm long with a 4 mmdiameter, would have a total light generation surface area ofAemit=16 bulbs×(π×4 mm)×820 mm=164,871 mm².

The ratio of the cumulative light source surface area to the outputsurface area, i.e., Parameter A, is a normalized and unitless measurerepresentative of a basic backlight challenge: transforming smallsurfaces of high brightness (and typically with a Lambertian emissionpattern) into a large surface output, preferably of relatively spatiallyuniform brightness, and preferably where the total luminous flux fromthe output surface is a substantial fraction of the total luminous fluxfrom the light sources (a fraction of 1.0 or 100% corresponding to anideal lossless system).

Our second parameter relates the average plan-view or lateral sourceaggregation (“SEP”) to the cavity depth (“H”). The cavity depth H(FIG. 1) is the physical distance from the back reflector to the frontreflector (output area Aout) along an axis perpendicular to the outputarea, i.e., it is the on-axis separation of the front and backreflectors. The cavity depth H can also be interpreted as the averageseparation between a non-planar output area and a non-planar backreflector. The average plan-view source separation SEP is a measure ofthe characteristic lateral spacing of light sources relative to theoutput surface Aout. The parameter SEP measures the degree to which thelight sources are disposed in a uniform spatial distribution within thecavity relative to the output surface. Larger values of the aggregationcharacteristic SEP indicate light sources are “clumped” or confinedwithin relatively small areas (volumes) of the cavity, while smallervalues of SEP indicate light-source spacings that are uniformly arrangedrelative to the output surface. In general, light sources within hollowcavities are arranged to provide as spatially uniform a distribution oflight flux on the output surface as is possible, resulting in minimumvalues of SEP for a given cavity geometry. Computation of SEP is bestexplained by examples.

FIG. 2 a shows a schematic plan view of a light source arrangement for adirect-lit backlight 30 a having eighteen light sources 32 a disposed onor proximate a back reflector 34 of transverse dimensions L (length) andW (width), where the associated front reflector and output area (notshown) have the same transverse dimensions and are coextensive with theback reflector 34. The sources 32 a are arranged in a regular repeatingpattern to form three equally spaced rows separated along the width ory-direction and six equally spaced columns separated along the length orx-direction (perpendicular to the y-direction). The average spacing ofthe sources along the x-direction is thus L/6, and the average spacingof the sources along the y-direction is W/3. The SEP is then calculatedas the average of these two orthogonal source spacings, orSEP=((L/6)+(W/3))/2.

For a 6×6 inch output area (L=W=6 inches or 153 mm), the SEP for thisexample becomes SEP=38 mm. Note that the result remains the same if thespacing between rows is not uniform, or if the spacing between columnsis not uniform, as long as the light sources are arranged in three rowsand three columns. SEP can be approximately evaluated for slightlyirregular spacing along rows and columns by assuming row and columnalignment.

Note also that each source 32 a may be a single emitting element such asa single white-emitting LED, or it may be the smallest unit cell orcluster of individual colored LEDs (e.g., red/green/blue orred/green/blue/green, etc.) that produces the desired backlight color,which is normally white light. In the case of a backlight designed toemit light of only one color, e.g., green, each source 32 a is a singlegreen-emitting LED.

FIG. 2 b shows a schematic plan view of a light source arrangement for abacklight 30 b similar to backlight 30 a, but where eighteen lightsources 32 b are disposed along a periphery of the back reflector 34 ina single line or column parallel to the y-direction. In this case thereis only one column of light sources 32 b along the length orx-direction, and 18 rows of (single) light sources 32 b arranged alongthe width or y-direction. The average spacing of the sources along thex-direction is thus L/1, and the average spacing of the sources alongthe y-direction is W/18. The SEP is again calculated as the average ofthese two orthogonal source spacings, orSEP=((L/1)+(W/18))/2.For a 6×6 inch output area (L=W=6 inches or 153 mm), the SEP for thisexample becomes SEP=81 mm. Note that the result remains the same if thespacing of the sources along the y-direction is not uniform. The SEPvalue of 81 mm is more than twice that of the FIG. 2 a embodiment (38mm), even though both embodiments use the same number of sources. Thisis as it should be, since each light source in the FIG. 2 b embodimentis required to influence or illuminate a much longer lateral dimensionalong the output surface than each source in FIG. 2 a, as the lightsources are more aggregated as compared with FIG. 2 a. SEP canalternatively be considered a transverse or lateral “radius ofinfluence” that each source, on average, is required to provide to theoutput area.

In a case were there is only one light source 32 a in the embodiment ofFIG. 2 a, or only one light source 32 b in the embodiment of FIG. 2 b,no matter where in relation to the output area the single light sourceis positioned, the average source spacing along the x-direction is L/1and the average source spacing along the y-direction is W/1, resultingin an SEP of ((L+W)/2) or 153 mm in the case of a 6×6 inch output area.

Linear-shaped light sources such as CCFLs that span substantially a fulltransverse dimension of the output area are treated differently thanlocalized or “point” sources like LEDs. FIG. 2 c shows a schematic planview of a light source arrangement for a direct-lit backlight 30 csimilar to backlight 30 a, but where six linear light sources 32 c arearranged in a linear array across the same back reflector 34 as shown.In this case the average spacing of the sources along the x-direction isL/6, because there are six sources distributed along the dimension L.The average spacing along the y-direction is zero because the lightsources 32 c are continuous along that direction. SEP is againcalculated as the average of these two orthogonal source spacings, orSEP=((L/6)+0)/2=L/12.This corresponds to half the average bulb-to-bulb spacing. For a 6×6inch output area (L=W=6 inches or 153 mm), the SEP for this examplebecomes SEP=13 mm.

With this background, we can characterize any recycling cavitybacklight, which has adequate brightness and spatial uniformity for itsintended application, by the two dimensionless parametersParameter A=Aemit/Aout; andParameter B=SEP/Hwhere Aemit, Aout, SEP, and H are as described above. FIG. 3 shows agraph that plots these two parameters as a backlight parameter space orbacklight design space.

This characterization is particularly straightforward for planarbacklight cavities, in which the back reflector (sometimes referred toherein as a backplane) of the backlight and the output area of thebacklight are both planar, parallel to each other, and of approximatelyequal area and approximately coextensive. Our two-parametercharacterization, however, is by no means restricted to plane parallelbacklight geometries, and may be generalized for any backlight geometryhaving the basic elements of an output surface associated with a frontreflector, and a back reflector that forms a light recycling cavity withthe front reflector, and a grouping of one or more light sourcesdisposed within, or optically connected to the cavity.

In addition, the determination of the characteristic light-sourcespacing parameter SEP has been illustrated above for light sources withsubstantially regular spacing along the x and y directions. Forinstances in which the light sources are disposed in an irregularpattern, SEP can be determined by the following equation:

${{SEP} = {{\frac{\left( {A_{out} - {\sum\limits_{{i = 1},N}A_{i}}} \right)}{A_{out}}\sqrt{\frac{A_{out}}{N}}} + {\sum\limits_{{i = 1},N}{\frac{A_{i}}{A_{out}}\sqrt{\frac{A_{i}}{\#_{i}}}}}}},$where A_(i) is the area of a circle i, circumscribing two or moreirregularly spaced light sources, the number of light sources in thecircumscribing circle being #_(i). N is the total number ofcircumscribing circles used to encompass all the light sources, wherethe number and location of all of the circumscribing circles, i throughN, are chosen so as to minimize the sum of all the circumscribingcircles' aggregate area,

$\sum\limits_{{i = 1},N}{A_{i}.}$

As an example, referring to FIG. 2 b, if the 18 light sources disposedin a single column along the y-direction were confined to the lower ½ ofthe y dimension, their spacing along the y-axis would be non-uniform,i.e., spaced at W/36 in the lower ½ of the axis, and no light sources inthe upper ½ of the axis. In this instance, use of the SEP equation forirregular spacing gives SEP=125 mm. So as the light sources are morespatially aggregated, the characteristic aggregation parameter, SEP,increases to 125 mm.

If the confinement or aggregation of the 18 light sources were tocontinue to occupy an ever smaller region along the y-axis, the SEPequation for irregular spacing will provide a larger dimensionapproaching a maximum of 153 mm, as describe above, for the instance ofa single light source residing in the 6×6 inch cavity.

FIG. 3 includes some description of general trends in the design space,which is self-explanatory. Also depicted is a point 36 representing ahypothetical initial backlight design. If the design is modified byreducing the cavity depth H but keeping all other design featuresconstant, the modified design will correspond to a point 36 a above andin vertical alignment with point 36. If the initial backlight design isinstead modified by replacing each individual source in the backlightwith a smaller emitting area source (e.g., replacing each LED die with asmaller LED die, but keeping the total number of LED dies constant andkeeping their spatial distribution the same), but keeping all otherdesign features constant, then the modified design will correspond to apoint 36 b to the left of and in horizontal alignment with point 36. Inyet another alternative, the initial design can be modified by addingmore light sources and arranging them more densely within the backlight,while keeping other design features constant. In this case the modifieddesign will correspond to a point 36 c that is both below and to theright of the starting point 36. One can anticipate that in the years tocome, LED sources may become brighter, and this may lead one to removelight sources from the initial design and arrange them more sparselywithin the backlight, keeping other design features constant. Such adesign modification would correspond to a point that is both above andto the left of the starting point 36.

Several commercially available LCD devices were obtained and theirbacklights analyzed with regard to the backlight parameter space. Theresulting design points are shown in the backlight design space graphillustrated in FIG. 4, which again plots Parameter A against ParameterB.

Points 40 a-d all represent commercial LCD televisions that utilizedirect-lit backlights powered by arrays of colored LEDs. Point 40 arepresents a Samsung Electronics 46 inch (diagonal measure) TV. Point 40b represents a 32 inch Sony LED TV, employing high-brightness OSRAMGolden Dragon LEDs. This unit grouped the LEDs in clusters of four(RGGB). Point 40 c represents another Sony 32 inch LED TV using OSRAMGolden Dragon LEDs, but this one grouped the LEDs in clusters of three:RGB. Point 40 d represents a Sony Qualia 40 inch TV. The points 40 a-dall have a Parameter B value of very nearly 2.

Points 40 e-f represent displays for commercial notebook computers.These each used an edge-lit backlight configuration, CCFL sources, andsolid (acrylic) light guides. Point 40 e represents an HP 14.1 inchdv1000, for Samsung LTN140W1-101. Point 40 f represents an AUO 15.4 inchnotebook computer, type B154-EW-02.

Points 40 g represent numerous commercial LCD televisions, each of whichused a direct-lit backlight illuminated with CCFLs.

Reviewing the plotted points in FIG. 4, one can see that backlightshaving the smallest values of Parameter A (points 40 a-d), i.e., thesmallest fraction of collective source emission area to backlight outputregion area, have relatively low values of Parameter B. Sources thathave a high value of Parameter B (points 40 e-f), i.e., thin cavitydepth in relation to the average source separation, utilize solid lightguides with their accompanying disadvantages, and achieve onlymoderately low values of Parameter A (since they utilize CCFL sources).

It would be desirable to provide a class of backlights having thincavities (e.g., Parameter B=3 or more), and having moderate to low oreven very low relative source areas (Parameter A=0.1 or less), andhaving a hollow cavity rather than a solid light guide.

As mentioned herein, Applicants have discovered combinations ofbacklight design features that are compatible with LED sourceillumination, and that can produce backlight designs that outperformexisting backlights in at least some respects. We will now discuss someof these backlight design features in more detail, and then, withreference to backlights that have been constructed and tested,demonstrate that such backlights (utilizing hollow cavity designs) arenow able to occupy a desirable space on the graph of FIG. 4.

We begin with a discussion of exemplary front and back reflectors. Inthis regard, reference is made generally to PCT Patent Publication No.WO2008/144656.

Exemplary partial reflectors (front reflectors) we describehere—particularly, for example, the asymmetric reflective films (ARFs)described in the 63274 Application—provide for low loss reflections andalso for better control of transmission and reflection of polarizedlight than is possible with TIR in a solid light guide alone. Thus, inaddition to improved light distribution in a lateral sense across theface of the display, the hollow light guide can also provide forimproved polarization control for large systems. Significant control oftransmission with angle of incidence is also possible with the preferredARFs mentioned above. In this manner, light from the mixing cavity canbe collimated to a significant degree, and a polarized light output froma single film construction can be provided.

Preferred front reflectors have a relatively high overall reflectivityto support relatively high recycling within the cavity. We characterizethis in terms of “hemispheric reflectivity,” meaning the totalreflectivity of a component (whether a surface, film, or collection offilms) when light is incident on it from all possible directions. Thus,the component is illuminated with light incident from all directions(and all polarization states, unless otherwise specified) within ahemisphere centered about a normal direction, and all light reflectedinto that same hemisphere is collected. The ratio of the total flux ofthe reflected light to the total flux of the incident light for awavelength range of interest yields the hemispheric reflectivity,R_(hemi). Characterizing a reflector in terms of its R_(hemi) isespecially convenient for recycling cavities because light is generallyincident on the internal surfaces of the cavity—whether the frontreflector, back reflector, or side reflectors—at all angles. Further,unlike the reflectivity for normal incidence, R_(hemi) is insensitiveto, and already takes into account, the variability of reflectivity withincidence angle, which may be very significant for some components(e.g., prismatic films).

In fact, in exemplary embodiments, preferred front reflectors exhibit a(direction-specific) reflectivity that increases with incidence angleaway from the normal (and a transmission that generally decreases withangle of incidence), at least for light incident in one plane. Suchreflective properties cause the light to be preferentially transmittedout of the front reflector at angles closer to the normal, i.e., closerto the viewing axis of the backlight. This helps to increase theperceived brightness of the display at viewing angles that are importantin the display industry (at the expense of lower perceived brightness athigher viewing angles, which are less common but as important). We saythat the increasing reflectivity with angle behavior is “at least forlight incident in one plane,” because sometimes a narrow viewing angleis desired for only one viewing plane, and a wider viewing angle isdesired in the orthogonal plane. An example is some LCD TV applications,where a wide viewing angle is desired for viewing in the horizontalplane, but a narrower viewing angle is specified for the vertical plane.In other cases, narrow angle viewing is desirable in both orthogonalplanes so as to maximize on-axis brightness.

When we discuss oblique angle reflectivity, it is helpful to keep inmind the geometrical considerations of FIG. 5. There, we see a surface50 that lies in an x-y plane, with a z-axis normal direction. If thesurface is a polarizing film or partially polarizing film (such as theARFs described in the 63274 Application), we designate for purposes ofthis application the y-axis as the “pass axis” and the x-axis as the“block axis.” In other words, if the film is a polarizing film, normallyincident light whose polarization axis is parallel to the y-axis ispreferentially transmitted compared to normally incident light whosepolarization axis is parallel to the x-axis. Of course, in general, thesurface 50 need not be a polarizing film.

Light can be incident on surface 50 from any direction, but weconcentrate on a first plane of incidence 52, parallel to the x-z plane,and a second plane of incidence 54, parallel to the y-z plane. “Plane ofincidence” of course refers to a plane containing the surface normal anda particular direction of light propagation. We show in the figure oneoblique light ray 53 incident in the plane 52, and another oblique lightray 55 incident in the plane 54. Assuming the light rays to beunpolarized, they will each have a polarization component that lies intheir respective planes of incidence (referred to as “p-polarized” lightand labeled “p” in the figure), and an orthogonal polarization componentthat is oriented perpendicular to the respective plane of incidence(referred to as “s-polarized light” and labeled “s” in the figure). Itis important to note that for polarizing surfaces, “s” and “p” can bealigned with either the pass axis or the block axis, depending on thedirection of the light ray. In the figure, the s-polarization componentof ray 53, and the p-polarization component of ray 55, are aligned withthe pass axis (the y-axis) and thus would be preferentially transmitted,while the opposite polarization components (p-polarization of ray 53,and s-polarization of ray 55) are aligned with the block axis.

With this in mind, let us consider the meaning of specifying (if wedesire) that the front reflector “exhibit a reflectivity that generallyincreases with angle of incidence,” in the case where the frontreflector is an ARF such as is described in the 63274 Application. TheARF includes a multilayer construction (e.g., coextruded polymermicrolayers that have been oriented under suitable conditions to producedesired refractive index relationships and desired reflectivitycharacteristics) having a very high reflectivity for normally incidentlight in the block polarization state and a lower but still substantialreflectivity (e.g., 25 to 90%) for normally incident light in the passpolarization state. The very high reflectivity of block-state light(p-polarized component of ray 53, and s-polarized component of ray 55)generally remains very high for all incidence angles. The moreinteresting behavior is for the pass-state light (s-polarized componentof ray 53, and p-polarized component of ray 55), since that exhibits anintermediate reflectivity at normal incidence. Oblique pass-state lightin the plane of incidence 52 will exhibit an increasing reflectivitywith increasing incidence angle due to the nature of s-polarized lightreflectivity (the relative amount of increase, however, will depend onthe initial value of pass-state reflectivity at normal incidence). Thus,light emitted from the ARF film in a viewing plane parallel to plane 52will be partially collimated or confined in angle. Oblique pass-statelight in the other plane of incidence 54 (i.e., the p-polarizedcomponent of ray 55), however, can exhibit any of three behaviorsdepending on the magnitude and polarity of the z-axis refractive indexdifference between microlayers relative to the in-plane refractive indexdifferences, as discussed in the 63274 Application.

In one case, a Brewster angle exists, and the reflectivity of this lightdecreases with increasing incidence angle. This produces bright off-axislobes in a viewing plane parallel to plane 54, which are usuallyundesirable in LCD viewing applications (although in other applicationsthis behavior may be acceptable, and even in the case of LCD viewingapplications this lobed output may be redirected towards the viewingaxis with the use of a prismatic film and such).

In another case, a Brewster angle does not exist or is very large, andthe reflectivity of the p-polarized light is relatively constant withincreasing incidence angle. This produces a relatively wide viewingangle in the referenced viewing plane.

In the third case, no Brewster angle exists, and the reflectivity of thep-polarized light increases significantly with incidence angle. This canproduce a relatively narrow viewing angle in the referenced viewingplane, where the degree of collimation is tailored at least in part bycontrolling the magnitude of the z-axis refractive index differencebetween microlayers in the ARF.

Of course, the reflective surface 50 need not have asymmetric on-axispolarizing properties as with ARF. Symmetric multilayer reflectors, forexample, can be designed to have a high reflectivity but withsubstantial transmission by appropriate choice of the number ofmicrolayers, layer thickness profile, refractive indices, and so forth.In such a case, the s-polarized components of both ray 53 and 55 willincrease with incidence angle, in the same manner with each other.Again, this is due to the nature of s-polarized light reflectivity, butthe relative amount of increase will depend on the initial value of thenormal incidence reflectivity. The p-polarized components of both ray 53and ray 55 will have the same angular behavior as each other, but thisbehavior can be controlled to be any of the three cases mentioned aboveby controlling the magnitude and polarity of the z-axis refractive indexdifference between microlayers relative to the in-plane refractive indexdifferences, as discussed in the 63274 Application.

Thus, we see that the increase in reflectivity with incidence angle (ifpresent) in the front reflector can refer to light of a useablepolarization state incident in a plane for which oblique light of theuseable polarization state is p-polarized. Alternately, such increase inreflectivity can refer to the average reflectivity of unpolarized lightin any plane of incidence.

Reflective (but partially transmissive) components other than thespecific ARF multilayer reflective films can also be used. Alternativecandidate materials include the following:

REFLECTOR TYPE CONSTRUCTION POLARIZATION Multilayer ¼ wave birefringentfilms, Polarizing asymmetric orientation ¼ wave birefringent films,Non-polarizing @ 0 symmetric orientation degrees Pile of platesbirefringent films, Polarizing asymmetric orientation ¼ wave isotropicfilms Non-polarizing @ 0 degrees Pile of plates films, isotropicNon-polarizing Perforated mirrors Non-polarizing Locally thinned partialreflectors Polarizing or non- (light transmission is increasedpolarizing in the thinned region) Crossed Reflective PolarizersPolarizing (angle of crossing controls amount of transmitted light)Metal Thin film enhanced metal films Non-polarizing Thin film enhancedmetal films, Non-polarizing perforated Wire grid Polarizing DiffusiveInorganic filled polymer films Non-polarizing Voided polymer filmsNon-polarizing Polymer foams Non-polarizing Polymer blendsNon-polarizing Polymer blends Polarizing Asymmetric DRPF MirrorsNon-polarizing Asymmetric DRPF Polarizers Polarizing Birefringentfibers - concentric Polarizing Islands-in-sea birefringent fibersPolarizing Holographic Diffusers Non-polarizing MicrostructuredLenticular structures or linear Non-polarizing prisms 2D structuredsurfaces (cube Non-polarizing corner, lenslet arrays, etc.) Cholesteric(with Lefthand Polarizing retarder films) Righthand PolarizingCombinations of both Polarizing - adjustable Metal/DielectricMetal/dielectric layered mirrors Non-polarizing

The above-mentioned reflectors can be used alone or in combination toprovide suitable front reflectors.

Preferred back reflectors also have a high hemispherical reflectivityfor visible light, typically, much higher than the front reflector,since the front reflector is deliberately designed to be partiallytransmissive to provide the required light output of the backlight.Reference is again made to the 63274 Application. The hemisphericalreflectivity of the back reflector is referred to as R^(b) _(hemi),while that of the front reflector is referred to as R^(f) _(hemi).Preferably, the product R^(f) _(hemi)*R^(b) _(hemi) is at least 70%(0.70), or 75%, or 80%.

Preferably, the front and/or back reflector can have a balance ofspecular and diffuse characteristics, thus having a semi-specularreflection characteristic as described more fully in commonly assignedPCT Patent Publication No. WO2008/144644.

Such reflectors have a transport ratio T of greater than 15% at a 15degree incidence angle and less than 95% at a 45 degree incidence angle,where T=(F−B)/(F+B), and F and B refer to forward-and backward-scatteredlight flux at a specified incidence angle. Incorporation of suchsemi-specular reflectors into the recycling cavity can provide adesirable balance between lateral transport and angular mixing of lightin the recycling cavity for optimal output uniformity at minimal cavitythicknesses. An example of a semi-specular reflector is Vikuiti™ ESRfilm that has been coated with a layer of beads.

Side reflectors are also typically included in the recycling cavity tominimize loss and enhance light propagation. As mentioned elsewhereherein, the side reflectors may be partitions that divide adjacentsections of a larger zoned backlight. Furthermore, the hollow nature ofthe preferred recycling cavities make them readily amenable tosubstantial design flexibility in the design of the side reflectors. Inone case, the side reflectors may simply be reflectorized walls of acake-pan-like support unit in a simple rectangular shape. Alternatively,the side reflectors may be strips of thin reflective film, whether aloneor applied to a somewhat stiffer substrate for mechanical support. Insuch cases, it is relatively easy to produce a cavity area that is otherthan rectangularly-shaped simply by bending one or more side-reflectorstrips into a desired shape.

This is shown in FIG. 6, where reference numeral 60 identifies abacklight output area of conventional rectangular design. An irregularlyshaped side reflector 62, formed for example by bending a strip ofreflective material and placing it between the (rectangular- orotherwise-shaped) front and back reflectors, produces an output areawith an irregular right edge. Other edges of the output area may besimilarly shaped to provide a wide variety of non-rectangular outputarea shapes, e.g., oval.

For illustrative purposes, it is convenient to further define theoptical surfaces of the backlight front reflector and back reflector,which form the recycling cavity. FIG. 7 is a schematic cross-sectionview of a display system 700 that includes a backlight 710 and an LCpanel 730. The backlight 710 is positioned to provide light to the LCpanel 730. The backlight 710 includes a front reflector 712 and a backreflector 714 that form a hollow light recycling cavity 716 having acavity depth H and an output region 718 of area Aout. The frontreflector 712 includes first, second, and third front reflector films720, 722, 724, that form the front reflector film stack. Any suitablefilms described herein can be utilized to provide the front reflector712.

The LC panel 730 typically includes a layer of LC 736 disposed betweenpanel plates 738. The plates 738 are often formed of glass and mayinclude electrode structures and alignment layers on their innersurfaces for controlling the orientation of the liquid crystals in theLC layer 736. These electrode structures are commonly arranged so as todefine LC panel pixels, i.e., areas of the LC layer where theorientation of the liquid crystals can be controlled independently ofadjacent areas. A color filter 740 may also be included with one or moreof the plates 738 for imposing color on the image displayed by the LCpanel 730.

The LC panel 730 is positioned between an upper absorbing polarizer 732and a lower absorbing polarizer 734. In the illustrated embodiment, theupper and lower absorbing polarizers 732, 734 are located outside the LCpanel 730. The absorbing polarizers 732, 734 and the LC panel 730 incombination control the transmission of light from a backlight 710through the display system 700 to the viewer. For example, the absorbingpolarizers 732, 734 may be arranged with their transmission axesperpendicular to each other. In an unactivated state, a pixel of the LClayer 736 may not change the polarization of light passing therethrough.Accordingly, light that passes through the lower absorbing polarizer 734is absorbed by the upper absorbing polarizer 732. When the pixel isactivated, the polarization of the light passing therethrough is rotatedso that at least some of the light that is transmitted through the lowerabsorbing polarizer 734 is also transmitted through the upper absorbingpolarizer 732. Selective activation of the different pixels of the LClayer 736, for example, by a controller (not shown), results in thelight passing out of the display system 700 at certain desiredlocations, thus forming an image seen by the viewer. The controller mayinclude, for example, a computer or a television controller thatreceives and displays television images.

One or more optional layers (not shown) may be provided proximate theupper absorbing polarizer 734, for example, to provide mechanical and/orenvironmental protection to the display surface. In one exemplaryembodiment, the layer may include a hardcoat over the upper absorbingpolarizer 734.

It will be appreciated that some types of LC displays may operate in amanner different from that described above. For example, the absorbingpolarizers 732, 734 may be aligned parallel and the LC panel may rotatethe polarization of the light when in an unactivated state. Regardless,the basic structure of such displays remains similar to that describedabove.

For modeling purposes in which we consider the front and back reflectorto be of substantially infinite extent, we can define a back reflectoreffective reflectivity for visible unpolarized light, “R^(b)_(hemi)(effective),” as including all of the reflective and lossyelements within the interior of the recycling cavity other than theaperture defining the output surface. In this regard, lossy elementssuch as LED dies, lenses, packaging, circuitry, and exposed circuitboard, are included in an area-fraction sense, with the surroundinghigh-reflectivity materials, to determine R^(b) _(hemi)(effective).Further, physical gaps between reflective surfaces are also included indefining this effective reflectivity. The physical location of thisR^(b) _(hemi)(effective) surface can then be conveniently drawn ascoincident with the mean surface of the physical cavity interior.

Further, it is convenient to define the optical properties of the frontreflector using the simple constructs R^(f) _(hemi), and T^(useable)(0deg), where “useable” (sometimes illustrated with the symbol “∥”) refersto the polarization state that is aligned with the pass-axis of thebottom absorbing polarizer 734 of an application LCD panel 730 (shownabove the backlight in FIG. 7).

R^(f) _(hemi) is a measurable quantity, describing the hemisphericalreflectivity of the front reflector. This front reflector can beconfigured to consist of a single reflective film or numerouscombinations of reflective films or reflective elements. They may belaminated or spaced apart, but in general they are defined as componentsthat are co-extensive with the output face of the cavity and operatetogether as a system to recycle light from the sources in order tothoroughly mix the light within the cavity. The components of the frontreflector can include diffusive elements such as diffuser plates, andsurface structure diffusers, as well as refractive elements such aslenticular and/or prismatic films.

The value of T^(useable)(0 deg) is defined as the ratio of thetransmitted intensity at 0 degrees (normal to the front reflector plane)with the front reflector and an absorbing polarizer overlaying anall-angle light source (e.g., an angle-mixed recycling cavity), to theintensity at 0 degrees for just the absorbing polarizer overlaying theall-angle light source. In instances where the display application isdesigned to accept light of some other set of angles other than thenormal angle, 0 degrees, or some arbitrary polarization state, thecharacteristic optical property of the front reflector can be moregenerally specified by T^(pol) (Ω), where Ω represents the solid angleof application acceptance of light from the output area of thebacklight, and “pol” refers to the polarization state of that light,which is required for application utility.

In reference to FIG. 7, it is further convenient to define the frontreflector 712 as a surface with properties R^(f) _(hemi), residing at aninner-most surface 726 of the front reflector film, or the inner-mostreflective component of the front reflector component stack, and T^(pol)(Ω) residing at an outer-most surface 728 of the front reflector film,or the outer-most reflective component of the front reflector componentstack. The backlight cavity depth H, can then be defined by theperpendicular distance from the R^(b) _(hemi)(effective) surface, andthe front reflector innermost surface 726 with property R^(f) _(hemi).For other arbitrary backlight cavity geometries, where the backreflector R^(b) _(hemi)(effective) surface 716 and the front reflectorR^(f) _(hemi) surface 726 are not co-planar, an effective cavity depthH_(eff) can be defined using appropriate geometrical constructs.

In many cases, it is desirable to combine high-recycling properties of abacklight cavity having a product R^(f) _(hemi)*R^(b) _(hemi)(effective)of at least 0.70, and preferably at least 0.80, and most preferably atleast 0.90, with sufficiently high values of T^(pol)(Ω), as thisprovides the angle-mixed and spatially mixed light within the cavity, anescape mechanism across the output area, for delivering spatiallyuniform brightness to the application.

In instances where the application requires light of a certainpolarization, such as an LCD panel, sufficiently high values ofT^(useable)(0 deg) may be needed to achieve high LCD-usable brightnessacross an application viewer-cone that is distributed about the normaldirection. Indeed, with the advent of new solid-state, high brightnessLED light sources, the dual challenge of transforming thehigh-brightness LED light generation surfaces into large-area, spatiallyuniform output surfaces of required brightness, without the loss ofsignificant portions of LED emitted light, has become formidable. Wetherefore describe herein hollow backlights with unique geometricalproperties SEP/H and Aemit/Aout that have adequate brightness andspatial uniformity for an intended application. This is achieved by thesurprising approach of employing backlight cavities with very highreflective surfaces front and back, in combination with a balance ofthese reflective surfaces' specular and diffuse characteristics, andwith light injection optics that partially collimate or confine lightinitially injected into the recycling cavity to propagation directionsclose to a transverse plane (the transverse plane being parallel to theoutput area of the backlight). In addition, we have found that by use ofunique front reflector T^(pol)(Ω) properties, high-applicationbrightness of application-usable polarization can be achieved.

To a good approximation, a suitably designed optical cavity in which alarge proportion of the light emitted by internal light sourcesundergoes multiple reflections between substantially coextensive frontand back reflectors, will have light rays within the cavity that canbecome substantially randomized in both direction and spatial locationwithin the cavity. The number of multiple reflections required toachieve this spatial and angular randomization of the light rays willdepend to a large extent on the specular and diffusive characteristicsfor the reflective elements (see, e.g., the 63032 Application).

For a recycling backlight cavity with a high degree of angular andspatial randomization of light rays within the cavity, the brightnessthrough the output surface into any particular output angle Ω will besubstantially equal at various points along the output surface. For sucha recycling cavity, the brightness into any particular output angle Ωcan be approximated by the expressionL(Ω)=((Light Source Lumens)/(2π×A _(out)))*(T ^(pol)(Ω)/(1−R ^(f)_(hemi) ×R ^(b) _(hemi)(effective))).The “Light Source Lumens” is that which is emitted into the cavity bythe light sources disposed within or optically coupled to the cavity.The expression T^(pol)(Ω)/(1−R^(f) _(hemi)×R^(b) _(hemi)(effective))represents the fractional increase in intensity into a solid angle Ω, ofpolarization “pol,” for the recycling cavity with front and backreflectors, compared with an angle-mixed flux into the forwardshemisphere (relative to the output surface) of the light sources alone.

We have found that LED light-source attributes and light injectiongeometries, in combination with novel high reflection materials, withappropriate front reflector transmission characteristics, can beconfigured to enable substantially hollow backlights in novel regions ofbacklight parameter space.

Component Characterization

We have measured R^(b) _(hemi) for several materials that have currentand potential uses as back reflector components. The measurementapparatus employed was custom-built by the Applicants but isstraightforward in design and operation. A commercial six inchintegrating sphere manufactured by Labsphere and made of Spectralon,with three mutually orthogonal ports, is used to illuminate samples andto determine hemispherical reflectance, R_(hemi), as well asnormal-angle transmittance T^(useable)(0 deg), for front reflector andback reflector samples. A stabilized light source illuminates the spherethrough one port. A PhotoResearch PR650 spectrophotometer is used tomeasure the sphere internal wall radiance through a second port. Thesample is placed on the third port. Calibration of the integratingsphere wall radiance is done by using a known reflectance standardplaced on the third port, and sphere-wall radiance is measured with andwithout the calibration standard. R_(hemi) is measured by placingsamples on the third port; sample hemispheric reflectance R_(hemi) isobtained by taking the ratio of the sphere wall radiance with andwithout the sample and employing a simple integrating spherebrightness-gain algorithm. This measurement of R_(hemi) is germane torecycling backlight cavity performance in that it is the all-angleinput, all-angle output reflection, measured in a way much like thatwhich occurs in an actual recycling cavity. Further, transmittance intoa chosen solid angle T(Ω), where Ω is defined by the collection apertureand its location relative to the normal to the sample surface, iscollected using the PhotoResearch PR650 spectrophotometer at the thirdport. The LCD-usable transmittance at normal angle T^(useable)(0 deg) isobtained by using the spectrophotometer at normal angle to the sampleand referencing the sample and overlaying absorbing polarizer (LCDdisplay polarizer SR5518, from San Ritz), to the absorbing polarizeralone.

Using the above described technique, R^(b) _(hemi) was determined forthe following materials:

TABLE I Reflection Ref letter Material characteristic R^(b) _(hemi) A 3MVikuiti ESR Specular 99.4% B MC-PET Near-Lambertian 98.4% diffuse C 3M2xTiPS Near-Lambertian 97.5% diffuse D 3M BGD ESR Semi-specular 98.0%

ESR is Vikuiti™ Enhanced Specular Reflector multilayer polymeric filmavailable from 3M Company. ESR had a hemispherical reflectivity of99.4%.

MC-PET is a microcellular PET reflective sheeting, available fromFurukawa America, Inc. (Peachtree City, Ga.). MCPET is diffuselyreflective.

2×TIPS is a porous polypropylene film having a high reflectivity and canbe made using thermally induced phase separation as described, e.g., inU.S. Pat. No. 5,976,686 (Kaytor et al.). Two sheets of TIPS werelaminated together using an optical adhesive to form a laminate. TheLambertian diffuse reflector had an average hemispherical reflectivityof 97.5%.

3M BDG ESR is an optical film that included a plurality of opticalelements coated onto an ESR film. The coating process includeddispersing a size distribution with geometric mean diameter of ˜18 μm,of small PMMA beads (MBX-20, available from Sekisui) into a solution ofIragacure 142437-73-01, IPA, and Cognis Photomer 6010. The solution wasmetered into a coater, and subsequently UV cured, producing a driedcoating thickness of approximately 40 μm. At this thickness, thedispersion of PMMA beads created a partial of hemispheric surfacestructure, randomly distributed spatially. The average radius ofprotrusion of the PMMA beads above the mean surface was estimated to beapproximately 60% of the average bead radius. The dried matrix wasformulated to have approximately the same refractive index as the PMMAbeads, minimizing the bulk scattering within the coating. BESR had ahemispherical reflectivity of 98.0%.

Further characterizations were performed using the techniques referencedabove on the materials potentially suitable for use as front reflectors,either single reflective films or combinations of reflective elementsand diffusive elements. The results of these characterizations arelisted in the following table:

TABLE II Reflection Material characteristic R^(f) _(hemi) T^(useable) (0deg) Astra DR55 Lambertian 43.9% 55.6% (DP) DP + DBEF-Q Lambertian 62.8%63.1% DP + ARF-37 Lambertian 73.2% 50.0% DP + 3xARF Lambertian 78.3%44.9% DP + GD + Lambertian 75.0% 59.0% BEF + DBEF ARF-89 Specular 92.5%11.0% ARF-86 + Specular 92.1% 12.8% BGD (afs) ARF-84 Specular 88.5%16.3% ARF-84 + Specular 88.5% 19.8% BGD (afs) ARF-68 Specular 83.2%31.6% ARF-37 Specular 67.6% 61.9% 3xARF Specular 75.4% 52.0% 4xARFSpecular 79.2% 44.4% 5xARF Specular 81.1% 39.6% 5xARF + BGDSemi-Specular 82.1 38.3% (ts) APF Specular 51.0 90.1% DBEF-DSemi-Specular 47.6 89.6%

89% R Asymmetric Reflective Film (ARF-89). This asymmetric reflectivefilm included 264 alternating microlayers of birefringent 90/10 coPENand non-birefringent PMMA. The 264 alternating microlayers were arrangedin a sequence of ¼ wave layer pairs, where the thickness gradient of thelayers was designed to provide a strong reflection resonance broadly anduniformly across bandwidth from approximately 400 nm to 900 nmwavelength for one polarization axis, and a weaker reflection resonancefor the orthogonal axis. Five micron thick skin layers of 90/10 coPENwere disposed on the outside surfaces of the coherent alteringmicrolayer stack. The overall thickness of the film, including thealternating microlayers, the PBLs and the skin layers, was approximately40 microns. This film was manufactured using the methods describedherein.

The birefringent refractive index values (measured at 633 nm) for the90/10 coPEN layers were nx1=1.785, ny1=1.685, nz1=1.518, and the indicesfor the PMMA layers were nx2=ny2=nz2=1.494.

ARF-89 had an average on-axis reflectivity of 89% in the pass axis, anaverage on-axis reflectivity of 98% in the block axis, and ahemispherical reflectivity of 92.5%.

84% R Asymmetric Reflective Film (ARF-84). This asymmetric reflectivefilm included 264 alternating microlayers of birefringent 90/10 coPENmaterial and non-birefringent PMMA material. The 264 alternatingmicrolayers were arranged in a sequence of ¼ wave layer pairs, where thethickness gradient of the layers was designed to provide a strongreflection resonance broadly and uniformly across a bandwidth fromapproximately 400 nm to 900 nm for one polarization axis, and a weakerreflection resonance for the orthogonal axis. Five micron thick skinlayers of 90/10 coPEN were disposed on the outside surfaces of thecoherent altering microlayer stack. The overall thickness of ARF-84,including the alternating microlayers, the PBLs and the skin layers, wasapproximately 40 μm. This film was manufactured using the methodsdescribed herein.

The birefringent refractive index values (measured at 633 nm) for thealternating microlayers of 90/10 coPEN were nx1=1.785, ny1=1.685, andnz1=1.518; and the indices for the microlayers of PMMA werenx2=ny2=nz2=1.494.

ARF-84 had an average on-axis reflectivity of 83.7% in the pass axis, anaverage on-axis reflectivity of 97.1% in the block axis, and ahemispherical reflectivity of 88.5%.

68% R Asymmetric Reflective Film (ARF-68). This asymmetric reflectivefilm included 274 alternating microlayers of birefringent 90/10 coPENmaterial and non-birefringent PMMA material. The 274 alternatingmicrolayers were arranged in a sequence of ¼ wave layer pairs, where thethickness gradient of the layers was designed to provide a strongreflection resonance broadly and uniformly across bandwidth fromapproximately 400 nm to 970 nm for one polarization axis, and a weakerreflection resonance for the orthogonal axis. Five micron thick skinlayers of a blend of 75% SA115 and 25% DP2554 were disposed on theoutside surfaces of the coherent altering microlayer stack. The overallthickness of the asymmetric reflective film, including the alternatingmicrolayers, the PBLs and the skin layers, was approximately 50 μm. Thisfilm was manufactured using the methods described herein.

The birefringent refractive index values for the alternating microlayersof 90/10 coPEN and of PMMA material were measured at 633 nm. The indicesfor the coPEN microlayers were nx1=1.820, ny1=1.615, and nz1=1.505. Theindex of refraction for the PMMA microlayers were nx2=ny2=nz2=1.494.

ARF-68 had an average on-axis reflectivity of 68.4% in the pass axis, anaverage on-axis reflectivity of 99.5% in the block axis, and ahemispherical reflectivity of 83.2%.

37% R Asymmetric Reflective Film (ARF-37). This asymmetric reflectivefilm included 274 alternating microlayers of birefringent 90/10 coPENand non-birefringent blend of CoPET-F and DP29341. The 274 alternatingmicrolayers were arranged in a sequence of ¼ wave layer pairs, where thethickness gradient of the layers was designed to provide a strongreflection resonance broadly and uniformly across a bandwidth fromapproximately 420 nm to 850 nm for one polarization axis, and a weakerreflection resonance for the orthogonal axis. Five micron thick skinlayers of coPEN 55/45/HD were disposed on the outside surfaces of thecoherent altering microlayer stack. The overall thickness of ARF-37,including the alternating microlayers, the PBLs and the skin layers, isapproximately 50 μm. This film was manufactured using the methodsdescribed herein.

The measured birefringent refractive index values (measured at 633 nm)for the alternating microlayers of 90/10 coPEN were nx1=1.820,ny1=1.615, and nz1=1.505, and the indices for the layers ofcoPET-F+DP29341 were nx2=ny2=nz2=1.542.

ARF-37 had an average on-axis reflectivity of 38.1% in the pass axis, anaverage on-axis reflectivity of 99.0% in the block axis, and ahemispherical reflectivity of 67.6%.

3 Layer Laminate of Asymmetric Reflective Film (3×ARF). This asymmetricreflective film included three asymmetric reflective films bondedtogether using two thick optical adhesive layers to form a laminate.Each film included 274 alternating microlayers of birefringent 90/10coPEN and non-birefringent of PET-G. The 274 alternating microlayerswere arranged in a sequence of ¼ wave layer pairs, where the thicknessgradient of the layers was designed to provide a strong reflectionresonance broadly and uniformly across a bandwidth from approximately410 nm to 940 nm for one polarization axis, and a weaker reflectionresonance for the orthogonal axis. There were no skin layers on theindividual multilayer optical films. Each film was manufactured usingthe methods described herein. The overall thickness of 2×ARF, includingthe alternating microlayers, PBLs and adhesive layers, was approximately100 μm. The birefringent refractive index values (measured at 633 nm)for the alternating microlayers of 90/10 coPEN were nx1=1.830,ny1=1.620, and nz1=1.500, and the indices for the microlayers of PET-Gwere nx2=ny2=nz2=1.563.

3×ARF had an average on-axis reflectivity of 48% in the pass axis, and ahemispherical reflectivity of 75.4%.

4 Layer Laminate of Asymmetric Reflective Film (4×ARF). This asymmetricreflective film included four asymmetric reflective films bondedtogether using three thick optical adhesive layers to form a laminate.Each film included 274 alternating microlayers of birefringent 90/10coPEN and non-birefringent of PET-G. The 274 alternating microlayerswere arranged in a sequence of ¼ wave layer pairs, where the thicknessgradient of the layers was designed to provide a strong reflectionresonance broadly and uniformly across a bandwidth from approximately410 nm to 940 nm for one polarization axis, and a weaker reflectionresonance for the orthogonal axis. There were no skin layers on theindividual multilayer optical films. Each film was manufactured usingthe methods described herein. The overall thickness of 4×ARF, includingthe alternating microlayers, PBLs and adhesive layers, was approximately200 μm.

The measured birefringent refractive index values (measured at 633 nm)for the alternating microlayers of 90/10 coPEN were nx1=1.830,ny1=1.620, and nz1=1.500, and the indices for the microlayers of PET-Gwere nx2=ny2=nz2=1.563.

4×ARF had an average on-axis reflectivity of 55.6% in the pass axis, anda hemispherical reflectivity of 79.2%.

5 Layer Laminate of Asymmetric Reflective Film (5×ARF). This multilayeroptical film is included four thick optical adhesive layers used to bondfive sheets of asymmetric reflective film in a laminate body. Each filmincluded 274 alternating microlayers of birefringent 90/10 coPEN andnon-birefringent of PET-G. The 274 alternating microlayers are arrangedin a sequence of ¼ wave layer pairs, where the thickness gradient of thelayers was designed to provide a strong reflection resonance broadly anduniformly across bandwidth from approximated 410 nm to 940 nm wavelengthfor one polarization axis, and a weaker reflection resonance for theorthogonal axis. There were no skin layers on the individual multilayeroptical films. The overall thickness of 5×ARF, including the alternatingmicrolayers, PBLs and adhesive layers, was approximately 260 μm. Themeasured (at 633 nm) birefringent refractive index values for thealternating microlayers of 90/10 coPEN material were nx1=1.830,ny1=1.620, and nz1=1.500, and the indices of the PET-G material werenx2=ny2=nz2=1.563.

In the following Examples, 5×ARF was used with an Opalus BS-702 beadedgain diffuser (available from Keiwa Corp.) laminated to the side of thesurface of the 5×ARF that faced the back reflector, such that the beads(i.e., microspheres) of the gain diffuser faced toward the backreflector.

5×ARF laminated to the beaded gain diffuser had an average on-axisreflectivity of 61.7% in the pass axis, and a hemispherical reflectivityof 81.1%.

BGD. Unless otherwise specified, some of the following Examples includedOpalus BS-702 beaded gain diffuser (available from Keiwa Corp.).

DBEF. Multilayer reflective polarizing film available from 3M Company.DBEF had a hemispheric reflectivity of 50.8%.

APF. Multilayer reflective polarizing film available from 3M Company.APF had a hemispheric reflectivity of 51.0%.

The recent emergence of very small-area light sources, such as LEDs,provides an opportunity to substantially increase the reflectivitylevels for a recycling backlight back reflector. Indeed, because LEDemission surface areas are so small compared to more conventional lightsources, such as CCFLs, the vast majority of a recycling cavity backreflector surface can be composed of materials with extremely highvalues of R^(b) _(hemi) such as those described in Table I above. Ofcourse, it is the R^(b) _(hemi)(effective) value for a recycling cavitythat operationally will determine how efficient the cavity is inrandomizing ray angles and creating a spatially uniform output-surfacebrightness. As stated above, the R^(b) _(hemi)(effective) value willinclude low reflectivity elements within the recycling cavity associatedwith light sources and electronics. We have characterized the effectivereflectivity of a Cree X-Lamp array consisting of small packaged dieRGGB LEDs, exposed circuitry, and an exposed local circuit board. Thevisible light reflectivity was characterized for the exposed area aroundand including the RGGB small die cluster, and the reflectivity wasestimated at ±50% on average across the visible band. We can thereforemake a reasonable assumption that R^(LED-area) _(hemi)=˜50%.

In the 63274 Application, the 66 clusters of RGB small package die werearrayed on the rectangular back surface of the recycling cavity, with 3M2×TIPS covering the majority of the back reflector surface, and specular3M ESR covering the rectangular box sidewalls. FIG. 8 depicts a top viewof an individual cluster, with dimensions given in millimeters. Carefulinspection of the geometrical arrangement of the 66 RGB die clusters,and the neighboring high reflectivity 2×TIPS material, indicated thatthe exposed area of material associated with the LED packaging andcircuitry was ˜11.2% of the back reflector area, with the remaining88.8% of the area covered by 2×TIPS. A simple area-fraction average ofthe R_(hemi) for each of the components yields a value for R^(b)_(hemi)(effective)=92.2%. This value for the recycling backlight's R^(b)_(hemi)(effective) can be validated by using the measured values shownin Table II for the front reflectors described in Examples C6, C7, C8,27, and 28 of the 63274 Application, where a complete description of theexamples can be found) and applying these measured values ofT^(useable)(0 deg) and R^(f) _(hemi) to the equation for LP) above. Inthis instance, the LED RGB cluster luminous output was assumed to be3.55 lumens/cluster.

FIG. 9 is referenced as indicating that the measured average luminance(0 deg) for each of the front reflector types in Examples C6, C7, C8,27, and 28 compares well with the assumption of R^(b)_(hemi)(effective)=92.2%, as compared with other values for R^(b)_(hemi)(effective), indicating that the calculation of R^(b)_(hemi)(effective) by area-fraction weighing of the back reflector R^(b)_(hemi) components is valid. FIG. 9 also demonstrates the great andsurprising impact that small changes in cavity efficiency (representedby the value of R^(b) _(hemi)(effective)) can have on backlightbrightness.

Other recycling backlight designs that take advantage of furtherreductions in the low-reflectivity material area-fraction of the backreflector can be quite advantageous in providing significantimprovements in light ray angular and spatial mixing, with lower lossesof light to the application through the front output surface. This isparticularly the case for recycling backlight architectures that employlarge-package LED dies in which die sizes are on the order of 1 mm-sq.In such an instance, the area-fraction of low reflectivity materialassociated with LED light source disposition within or along theperimeter of the recycling cavity can be significantly diminished.Depending on the choice of back reflector components, the value of R^(b)_(hemi)(effective) can exceed 96%, and preferably 97%, and morepreferably 98%.

Preface to Examples Uniformity

A wide assortment of backlights were constructed and tested as providedfurther below. In most cases, both the average brightness and anindication of uniformity of each backlight is provided. These resultsare provided so that one can assess, at least to some extent, whetherthe given backlight may be suitable for any particular intendedapplication, and not necessarily limited to applications for LCD TVs orsimilar end-use devices.

As used herein, therefore, the term “acceptable spatial uniformity”refers to both acceptable uniformity of both overall intensity andcolor. What is considered acceptable brightness and spatial uniformitydepends upon the particular application for which the backlight will beused. For example, a common reference standard for LCD uniformity is TCO05 (The Swedish Confederation of Professional Employees, version 2.0,Sep. 21, 2005, p. 9), which specifies an acceptance threshold luminanceratio of greater than 66%. In the early commercialization of aparticular technology, uniformity standards may be lower; for example,when notebook computers were first introduced, acceptable uniformity wasin the range of 50-60%. Further, for example, internally illuminatedchannel letters are another application where luminance uniformity is animportant performance metric. Here, human factor studies have shown thatmost people judge channel letter uniformity as being acceptable if theluminance ratio is greater than 50%. See, e.g., Freyssinier et al.,Evaluation of light emitting diodes for signage applications, ThirdInternational Conference of Solid State Lighting, Proceedings of SPIE5187:309-317 (2004). Emergency signage is yet another ubiquitousapplication for light emitting panels. An example specification foruniformity is the Energy Star program for Exit Signs. See Energy StarProgram Requirements for Exit Signs Draft 1, Eligibility CriteriaVersion 3.0. For an exit sign to qualify for Energy Star designation,the sign should have a luminance uniformity of greater than 20:1 (i.e.,5%).

The Video Electronics Standards Association (VESA) sets guidelines forluminance and color uniformity in their publication Flat Panel DisplayMeasurements Standard, v. 2.0 (published Jun. 1, 2001) standard 306-1Sampled Uniformity and Color of White (herein referred to as VESA 9ptColor Nonuniformity Standard). The VESA 9pt Luminance Uniformityreported herein is determined from 9 specified circular regions(referred to as “sample points”) on the output surface of the backlightas

${{VESA}\mspace{14mu} 9\mspace{14mu}{pt}\mspace{14mu}{Luminan}\;{ce}\mspace{14mu}{Uniformity}} = \frac{L_{\min}}{L_{\max}}$

where L_(min) is the minimum value of the luminance of the 9 points andL_(max) is the maximum value of the luminance of the 9 points. Highervalues of VESA 9pt Luminance Uniformity indicate systems that are moreuniform.

The VESA 9pt Color Nonuniformity is determined as the largest value ofthe color difference between any two pairs of the 9 sample points. Thecolor difference Δu′v′ isΔu′v′=√{square root over ((u ₁ ′−u ₂′)²+(v ₁ ′−v ₂′)²)}{square root over((u ₁ ′−u ₂′)²+(v ₁ ′−v ₂′)²)}

where the subscripts 1 and 2 denote the two sample points beingcompared. Lower values of VESA 9pt Color Nonuniformity indicate systemsthat are more uniform.

Backlight Examples

A wide assortment of backlights were constructed and tested. The detailsof construction (including backlight geometry, reflective materials andother optical materials used, light sources used and theirconfiguration, and other significant backlight components), testmethodology, and results are provided in the 63274 Application citedabove, and all such information is incorporated herein in its entirety.That application uses the following designation for the variousembodiments constructed, and this designation is followed in the presentapplication:

C1, C2, C3, C4, C5, C6, C7, C8; and

1, 2, 3, 4, 5, 6a through 6f, 7, 8, 9, 10a, 10b, 11a, 11b, 12a through12f, and 13 through 31.

As will be shown below, the examples provide numerous illustrations ofhollow light-recycling cavities that reside in the desired cavity designspace discussed above, and provide at least adequate brightness anduniformity characteristics. Further, the examples demonstrate the effectof different combinations of reflective films for the front and backreflectors. Different light source arrangements are also included, withsome being edge- and others direct-lit type. Large-area edge-litbacklights (ranging at least from 12 to 40 inch diagonal measure) arealso included. Some of the examples demonstrate the effect of turningoff selected light sources, showing in some cases the robustness of thedesign to light source failure or burnout. Finally, the examples invarious combinations are submitted to demonstrate the properties calledout in the claims below.

Additional Examples 21a through 21h

Some additional examples were carried out as follows. These additionalexamples used the same physical layout as Examples 20 and 21, exceptthat in some cases the ARF-89 film used as the front reflector wasreplaced with 3M APF reflective polarizing film, and variouscombinations of light sources were turned on or off to demonstratesensitivity to light source burnout. We refer to these additionalexamples as Examples 21a, b, c, d, e, f.

The backlight system from Examples 20 and 21 was used for these exampleswith the only difference being that one of the green LEDs was turnedoff. The green LED that was off was located at the left side of the leftbank of LEDs when the box was viewed from the output side with the LEDbar located along the top.

Example 21a: the output region of the Edge-Lit, Hollow Backlight wascovered by a reflective polarizer (APF mounted on an acrylic plate)which was placed over the output area of the backlight. All of the LEDs(red, green, and blue, except for the single green LED mentioned above,for a total of 4R 7G 4B) were turned on to produce white light. Thebacklight was bright close to the LEDs and visibly darker at the far end(away from the LEDs).

Example 21b: the same configuration as for Example 21a was used, butonly the green LEDs were powered (except for the single green LEDmentioned above, for a total of 7G). The backlight appeared muchbrighter close to the LEDs and darker at the far end (away from theLEDs).

Example 21c: the same configuration as for Example 21a was used, butonly the four green LEDs of the right bank of LEDs were powered (for atotal of 4G). The backlight appeared much brighter on the right sideclose to the LEDs and was darker at the far end (away from the LEDs) andalso darker on the left side where the LEDs weren't lit.

Example 21d: the same configuration as for Example 21a was used, butonly the three green LEDs of the left bank of LEDs were powered (for atotal of 3G). The backlight appeared much brighter on the left sideclose to the LEDs and was darker at the far end (away from the LEDs) andalso darker on the right side where the LEDs weren't lit.

Example 21e: the output region of the Edge-Lit, Hollow Backlight wascovered by a partial reflector (ARF-89 mounted on an acrylic plate)which was placed over the output area of the backlight. The partialreflector had a pass-axis transmission of approximately 11% for visiblelight. The back of the backlight was covered with Bead-coated ESR. Allof the LEDs were turned on (red, green, and blue, except for the singlegreen LED mentioned above, for a total of 4R 7G 4B) to produce whitelight. The backlight appeared uniformly illuminated.

Example 21f: the same configuration as for Example 21e was used, exceptonly the green LEDs were powered (except for the single green LEDmentioned above, for a total of 7G). The backlight appeared uniformlyilluminated.

Example 21g: the same configuration as for Example 21e was used, exceptthat only the four green LEDs of the right bank of LEDs were powered(for a total of 4G). The backlight appeared uniformly illuminated.

Example 21h: the same configuration as for Example 21e was used, exceptthat only the three green LEDs of the left bank of LEDs were powered.The backlight appeared uniformly illuminated.

The measurement results for these examples are summarized below:

Y_Uniformity Example Y_avg Y_std [VESA9] 21a 3206 26.8% 52.8% 21b 195326.3% 52.8% 21c 1107 32.3% 39.7% 21d 853 33.5% 40.4% 21e 2788 7.8% 91.0%21f 1748 8.2% 86.6% 21g 998 9.1% 78.7% 21h 768 9.2% 80.6%

From all of the foregoing examples, we have sufficient information tocalculate the backlight design parameters: Parameter A, equal toAemit/Aout; and Parameter B, equal to SEP/H.

The edge-lit examples are plotted in FIG. 10, and the direct-litexamples are plotted in FIG. 11. In both cases, the labels used in thefigures correspond to the Example numbering convention above.

The various embodiments of backlights described herein can include alight sensor and feedback system to detect and control one or both ofthe brightness and color of light from the light sources. For example, asensor can be located near individual light sources or clusters ofsources to monitor output and provide feedback to control, maintain, oradjust a white point or color temperature. It may be beneficial tolocate one or more sensors along an edge or within the cavity to samplethe mixed light. In some instances it may be beneficial to provide asensor to detect ambient light outside the display in the viewingenvironment, for example, the room that the display is in. Control logiccan be used to appropriately adjust the output of the light sourcesbased on ambient viewing conditions. Any suitable sensor or sensors canbe used, e.g., light-to-frequency or light-to-voltage sensors (availablefrom Texas Advanced Optoelectronic Solutions, Plano, Tex.).Additionally, thermal sensors can be used to monitor and control theoutput of light sources. Any of these techniques can be used to adjustlight output based on operating conditions and compensation forcomponent aging over time. Further, sensors can be used for dynamiccontrast, vertical scanning or horizontal zones, or field sequentialsystems to supply feedback signals to the control system.

Unless otherwise indicated, references to “backlights” are also intendedto apply to other extended area lighting devices that provide nominallyuniform illumination in their intended application. Such other devicesmay provide either polarized or unpolarized outputs. Examples includelight boxes, light emitting panels, signs, channel letters, visibilitylights, e.g., for cars or motorcycles, and general illumination devicesdesigned for indoor (e.g. home or office) or outdoor use, sometimesreferred to as “luminaires.” Note also that edge-lit devices can beconfigured to emit light out of both opposed major surfaces—i.e., bothout of the “front reflector” and “back reflector” referred to above—inwhich case both the front and back reflectors are partiallytransmissive. Such a device can illuminate two independent LCD panels orother graphic members placed on opposite sides of the backlight. In thatcase the front and back reflectors may be of the same or similarconstruction.

The term “LED” refers to a diode that emits light, whether visible,ultraviolet, or infrared. It includes incoherent encased or encapsulatedsemiconductor devices marketed as “LEDs,” whether of the conventional orsuper radiant variety. If the LED emits non-visible light such asultraviolet light, and in some cases where it emits visible light, it ispackaged to include a phosphor (or it may illuminate a remotely disposedphosphor) to convert short wavelength light to longer wavelength visiblelight, in some cases yielding a device that emits white light. An “LEDdie” is an LED in its most basic form, i.e., in the form of anindividual component or chip made by semiconductor processingprocedures. The component or chip can include electrical contactssuitable for application of power to energize the device. The individuallayers and other functional elements of the component or chip aretypically formed on the wafer scale, and the finished wafer can then bediced into individual piece parts to yield a multiplicity of LED dies.An LED may also include a cup-shaped reflector or other reflectivesubstrate, encapsulating material formed into a simple dome-shaped lensor any other known shape or structure, extractor(s), and other packagingelements, which elements may be used to produce a forward-emitting,side-emitting, or other desired light output distribution.

Unless otherwise indicated, references to LEDs are also intended toapply to other sources capable of emitting bright light, whether coloredor white, and whether polarized or unpolarized, in a small emittingarea. Examples include semiconductor laser devices, and sources thatutilize solid state laser pumping.

In some embodiments, brightness uniformity of an edge-lit backlight canbe enhanced by aiming the injection light output direction, adjustingspacing between adjacent light sources or groups of light sources, or acombination of the two techniques. For example, forward emitting lightsources having narrow light distribution cone angles as described hereincan be selected as a way of controlling the direction of light emittedby the light sources. Typically, for edge-lit backlights, the lightsources can be arranged along one or more edges of a backlight such thatthe emitted beams are directed substantially perpendicular to the inputedge or edges and parallel to each other. By aiming the beams of one ormore light sources in a non-perpendicular direction and towards selectedareas of the backlight, the brightness of the selected area can beincreased with a corresponding brightness decrease in other areas. Forexample, in a backlight having light sources uniformly disposed alongone edge, the light sources can be aimed such that all of the beamsintersect at the approximate center of the backlight, thus resulting ina bright center and less bright edges. If fewer than all of the beamsare directed to intersect at the center, the center brightness can bedecreased, thereby providing a mechanism to adjust the brightness to adesired level. Analogous arrangements can be used to produce, forexample, brighter edges and a less bright center. Any suitable techniquecan be used to control the emission direction of the light sources,e.g., mounting orientation of the light sources, lenses, extractors,collimating reflectors, etc.

Light sources can be arranged along one or more edges of a backlightsuch that spacing between them is non-uniform. In this situation, thepart of the backlight having more closely spaced light sources will tendto be brighter. For example, in a backlight having 40 LEDs disposedalong one edge, the center 20 LEDs can be more closely spaced than theflanking 10 LEDs towards each edge, thus producing a brighter center.Analogous adjustments can be used to produce brighter edges.

This non-uniform spacing between light sources can also be provided bycontrolling the light output of individual or groups of light sources tosimulate a non-uniform physical spacing. For example, one or more lightsources can be powered down or turned off to control the distribution ofinjected light.

Other suitable techniques can be used either alone or in combinationwith light source aiming and distribution to provide a desired outputlight distribution. For example, uniform or non-uniform patterns oflocalized extraction structures can be provided on one or both of thefront reflector and back reflector to redirect some of light out of thebacklight.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified by the term “about.” Accordingly,unless indicated to the contrary, the numerical parameters set forth inthe foregoing specification and attached claims are approximations thatcan vary depending upon the desired properties sought to be obtained bythose skilled in the art utilizing the teachings disclosed herein.

Various modifications and alterations of this disclosure will beapparent to those skilled in the art without departing from the scopeand spirit of this disclosure, and it should be understood that thisdisclosure is not limited to the illustrative embodiments set forthherein. All U.S. patents, patent application publications, unpublishedpatent applications, and other patent and non-patent documents referredto herein are incorporated by reference in their entireties, except tothe extent any subject matter therein directly contradicts the foregoingdisclosure.

What is claimed is:
 1. An edge-lit backlight, comprising: a front andback reflector forming a hollow light recycling cavity having a cavitydepth H and an output region of area Aout; and one or more light sourcesdisposed proximate a periphery of the backlight to emit light into thelight recycling cavity, the light sources having an average plan viewsource separation of SEP and collectively having an active emitting areaAemit; wherein a first parameter equals Aemit/Aout; wherein a secondparameter equals SEP/H; wherein the backlight is characterized by thefirst parameter being in a range from 0.0001 to 0.1, and by the secondparameter being in a range from 3 to 10; and wherein the front reflectorhas a hemispherical reflectivity for unpolarized visible light of R^(f)_(hemi), and the back reflector has a hemispherical reflectivity forunpolarized visible light of R^(b) _(hemi), and R^(f) _(hemi)*R^(b)_(hemi) is at least 0.70.
 2. The backlight of claim 1, wherein the oneor more light sources comprise one or more LEDs.
 3. The backlight ofclaim 1, wherein R^(f) _(hemi)*R^(b) _(hemi) is at least 0.80.
 4. Thebacklight of claim 1, wherein the backlight exhibits a VESA 9 uniformityvalue of at least 50% over the output region.
 5. The backlight of claim4, wherein the backlight exhibits a VESA 9 uniformity value of at least60%.
 6. The backlight of claim 5, wherein the backlight exhibits a VESA9 uniformity value of at least 70%.
 7. The backlight of claim 5, whereinthe backlight exhibits a VESA 9 uniformity value of at least 80%.
 8. Thebacklight of claim 1, wherein the output region is generally rectangularand has a diagonal measure of at least 12 inches.
 9. The backlight ofclaim 8, wherein the diagonal measure is at least 23 inches.
 10. Thebacklight of claim 9, wherein the diagonal measure is at least 40inches.
 11. An edge-lit backlight, comprising: a front and backreflector forming a hollow light recycling cavity having a cavity depthH and an output region of area Aout; and a number N of one or more lightsources disposed proximate a periphery of the light recycling cavity andemitting light into the light recycling cavity; wherein the outputregion is generally rectangular in shape and has a diagonal measure ofat least 30 inches; wherein the output region exhibits a VESA 9uniformity value of at least 50%; wherein the one or more light sourceshave an average plan view source separation of SEP and collectively havean active light emitting area Aemit; wherein a first parameter equalsAemit/Aout; wherein a second parameter equals SEP/H; and wherein thebacklight is characterized by the first parameter being in a range form0.0001 to 0.1, and by the second parameter being in a range from 3 to10.
 12. The backlight of claim 11, wherein the one or more light sourcescomprise one or more LEDs.
 13. The backlight of claim 11, wherein theoutput region exhibits a VESA 9 uniformity value of at least 70%.
 14. Abacklight, comprising: a front and back reflector forming a hollow lightrecycling cavity having an output region; and a number N of lightsources disposed to emit light into the light recycling cavity, the Nlight sources including a subset of M light sources that are adjacent toeach other, where M is at least 10% of N, or is at least 2, or both;wherein the backlight exhibits a VESA brightness uniformity value overits output region of at least 50% both when all N light sources areenergized and when all of the M light sources are selectively turnedoff; and wherein the front reflector has a hemispherical reflectivityfor unpolarized visible light of R^(f) _(hemi), and the back reflectorhas a hemispherical reflectivity for unpolarized visible light of R^(b)_(hemi), and R^(f) _(hemi) * R^(b) _(hemi) is at least 0.70.
 15. Thebacklight of claim 14, wherein the recycling cavity has a depth H;wherein the output region has an area Aout; wherein the N light sourceshave an average plan view source separation of SEP and collectively havean active emitting area Aemit; wherein a first parameter equalsAemit/Aout; wherein a second parameter equals SEP/H; and wherein thebacklight is characterized by the first parameter being in a range from0.0001 to 0.1, and by the second parameter being in a range from 3 to10.
 16. The backlight of claim 14, wherein the front and back reflectorsare each substantially spatially uniform.
 17. The backlight of claim 14,wherein the N light sources are predominantly disposed proximate aperiphery of the output region to provide an edge-lit backlight.